You mean you want to get an 18 hour day from a timer? Everyone has the same time, 24 hours in a day so you will have to have the same hours in a day that I do. Sorry, no timer will change that.
If you want a timer that will divide your day like that, pretty much any timer will allow you to do 6 on, 12 off, and 6 on.
I am sure I am not understanding your question, but as written, my answer number one is correct.